It has heretofore been proposed, for example as shown in U.S. Pat. Nos. 2,352,982 and 3,195,156, to form self-tapping screws having a polygonal cross-section throughout the length of the externally threaded shank so that the turns of the thread have circumferentially spaced lobes and intermediate sides that are radially relieved relative to the lobes such that the lobes form an internal thread in an opening in a workpiece by a swaging action, as contrasted to a cutting action. The lobular cross-sectional configuration of the screw having intermediate radially relieved sides provides space for the material of the workpiece to flow both circumferentially and axially during swaging of the internal threads in the workpiece, and the intermediate radially relieved sides reduce the area of frictional contact between the screw threads and the workpiece so as to generally reduce the driving torque required to drive the screw into the workpiece. When screws made in accordance with the above patents are driven into a hole in a relatively thin strap or strip of material, the driving torque reaches a maximum after the screw has advanced into the workpiece to substantially fully form the thread in the hole and then drops off somewhat. However, the torque required to continue advance of the screw through the hole remains relatively high. There are applications where it is very desirable to provide a screw having a very low run-in torque or prevailing torque, and it is desirable that this low run-in torque be reached within the first or second revolution after maximum driving torque or as close to the point end of the screw as possible. A typical example of such an application is an electrical terminal screw. The electrical terminal screw requires a thread forming screw with low run-in torque that does not generate chips since the presence of metallic chips would adversely affect most electrical equipment. Moreover, a low prevailing torque is desired so that the limited torque available for tightening the screw can be advantageously used to provide increased clamping force instead of merely overcoming resistance to turning of the screw in the workpiece.
When screws made in accordance with the aforementioned patents are driven into a long or blind hole such as in a casting, the driving torque first builds up rapidly as the screw swages the initial threads adjacent the inlet of the hole, and the driving torque continues to build up because the work entering end of the screw continues to swage a thread in the hole while the shank portion of the screw has a progressively increasing number of threads in frictional contact with the internal threads swaged by the work entering end of the screw. The continued build-up in driving torque is undesirable not only when manually driving the screws, but also when using clutch controlled power drivers. Such power drivers must be set sufficiently high to overcome the maximum driving torque of the screw and yet sufficiently low to prevent stripping or breading of the screw. It is accordingly desirable to maintain the maximum driving torque required for fully driving the screw into the workpiece as low and as uniform as possible to avoid the necessity of critical setting of the clutches on the power drivers.